surely all you need to do is establish the mid point for the most inward focusing eyepiece and most outward focusing eyepiece and then buy or use an extension tube that allows for this difference within the 45mm of travel? e.g. the difference between the most inward position (7mm BGO) and outward position (26mm Nagler) is about 18mm. in your shoes, I'd set the focuser at say 10mm then establish how much extension your most inward focusing eyepiece needs. then do the same with your most outward focusing eyepiece (this will be easier with someone's help, for the measuring, possibly). if the differece is less then 35mm (the difference between 10mm out already and max outward position) then the length of extension for the most inward focusing eyepiece to achieve focus will be fine for all eyepieces. you'll then just need the one extension which can always stay in the scope. hopefully this will be a standard extension distance (35mm, 50mm or 80mm, or even your 120mm supplied with the scope). another option might be to do the above process but use a baader fine tuning ring (28mm perhaps) to allow some additional small extension to your 120mmtube to achieve the same result. hope this is clear but feel free to seek clarification.